To improve performance of a rotating machine, such as a turbine, the gap (tip clearance) between the rotary part (for example, the blades) and the stationary part (for example, the casing) needs to be small. Methods to measure the clearance include optical clearance measurement. In the optical clearance measurement, passing timings of the blades or an reflective target are detected, and the time difference information is used for calculation in accordance with the triangulation method.
In the optical clearance measurement, a light sensor 60 using optical fibers as illustrated in FIG. 5A is used. This light sensor 60 has a pair of optical fiber bundles 61 and 62 slanted to form two sides of a triangle in triangulation (see FIG. 6 described later). As illustrated in FIG. 5B, the optical fiber bundle 61 includes one light-emitting fiber 61a that emits light and multiple light-receiving fibers 61b that receive light. The one light-emitting fiber 61a is positioned at the center, around which are arranged multiple light-receiving fibers 61b (Patent Document 1). The optical fiber bundle 62 also has the same structure as that of the optical fiber bundle 61, including a light-emitting fiber 62a and light-receiving fibers 62b. 
The measurement principle of the above light sensor 60 will be described with reference to FIGS. 6 and 7. Here, assume that the distal end portion of a blade 71 which serves as the reflection target is measured. During measurement, light is emitted from each of the light-emitting fiber 61a in the optical fiber bundle 61 and the light-emitting fiber 62a in the optical fiber bundle 62. When the rotating blade 71 passes by position C, the light-receiving fibers 61b in the optical fiber bundle 61 receive light reflected from the blade 71 at time t1. After that, when the rotating blade 71 passes by position D, the light-receiving fibers 62b in the optical fiber bundle 62 receive light reflected from the blade 71 at time t2.
In this way, when the blade 71 passes by two light beams from the pair of light-emitting fibers 61a and 62a, two waveforms (changes in intensity of the reflected light) shifted from each other by time difference Δt are obtained as illustrated in the graph of FIG. 7. From the two waveforms, time difference Δt is calculated.
Here, in FIG. 6, the distance from the distal end of the light sensor 60 to the distal end of the blade 71, in other words, the clearance is represented by d. The angle formed by two light beams outputted from the pair of light-emitting fibers 61a and 62a is represented by α. The interval between the distal ends of the pair of light-emitting fibers 61a and 62a, in other words, interval AB is represented by L. The peripheral speed of the blade 71 is represented by v; the rotation radius of the blade 71, R; and the revolution speed of the blade 71, N. Then, interval CD is expressed as v×Δt=2×R×π×N×Δt, and clearance d can be calculated by the following formula 1. Note that revolution speed N is acquired separately, using a pulse meter or the like.
                              [                      Math            .                                                  ⁢            1                    ]                ⁢                                                                                      d        =                                                                              1                  2                                ⁢                                  CD                  _                                                            tan                ⁢                                  α                  2                                                      -                                                            1                  2                                ⁢                                  AB                  _                                                            tan                ⁢                                  α                  2                                                              =                                                                                          1                    2                                    ×                  v                  ×                  Δ                  ⁢                                                                          ⁢                  t                                                  tan                  ⁢                                      α                    2                                                              -                                                                    1                    2                                    ×                  L                                                  tan                  ⁢                                      α                    2                                                                        =                                                                                1                    2                                    ×                  2                  ×                  R                  ×                  π                  ×                  N                  ×                  Δ                  ⁢                                                                          ⁢                  t                                                  tan                  ⁢                                      α                    2                                                              -                                                                    1                    2                                    ×                  L                                                  tan                  ⁢                                      α                    2                                                                                                          (                  Formula          ⁢                                          ⁢          1                )            
Specifically, “angle α”, “interval AB”, and “interval CD” are parameters necessary for triangulation and correspond to “the angle of the small triangle”, “the base of the small triangle”, and “the base of the large triangle”, respectively, as illustrated in FIG. 8. Since angle α and interval AB are known parameters, interval CD can be calculated from time difference Δt between the two waveforms, and clearance d can be calculated as a result.